I believe systemd-networkd has its own implementation of DHCP and therefore doesn't use dhclient. But I wonder if it's behavior is any better in this respect.
systemd-networkd keeps open that kind of socket for LLDP but apparently not for the DHCP client code. wpa_supplicant also keeps open this type of socket on my local system. and the dhcpd daemons on my routers have some of those too for each interface...
i wonder if the slow path here could be avoided by using separate network namespaces in a way these sockets don't even get to see the packets...
This has piqued my interest.